Performance comparisons between semiconductor and fiber amplifier gain assistance in a recirculating frequency shifter

Recirculating frequency shifter (RFS) loops can be used for electronically programmable, variable-spacing multiline spectrum generation, which can benefit the development of fully flexible optical communications and other frequency comb applications. Here, we report on and explain the observation of significant performance variations between chip-based gain in semiconductor optical amplifiers (SOA) and fiber-based gain in erbium-doped fiber amplifiers (EDFA) when used as the gain element in the RFS. Previously, SOAs and EDFAs have been demonstrated in different RFS experiments and studied separately from each other; thus, discussion mainly focused on the noise from amplified spontaneous emission. We show that SOA effects, including four wave mixing, can be measured, which impose limits to the wavelength spacing of the combs, and that this effect is mitigated by increasing the RF drive frequency of the RFS and operating SOA in deeper saturation.